How can I add a recurring donation button to my existing donation form?

Inside of this guide you can find how to setup a recurring payment option:

http://www.jotform.com/help/166-How-to-setup-a-recurring-payment-option 

Unfortunately, we don't have a feature that would allow you to setup a recurring donation with a custom amount in a form. 

But Paypal will allow you to do it using their workaround here:

https://www.paypal.com/webapps/mpp/get-started/create-recurring-donation-button